The present invention is directed toward an improved process for catalytic reforming. More particularly, the described inventive technique is adaptable for utilization in a low pressure catalytic reforming process.
Various types of catalytic hydrocarbon conversion reaction systems have found widespread utilization throughout the petroleum and petrochemical industries for effecting the conversion of hydrocarbons to a multitudinous number of products. Reactions employed in such systems often result in either the net production of hydrogen or the net consumption of hydrogen. Typical of the net hydrogen-producing hydrocarbon reaction systems are catalytic reforming, catalytic dehydrogenation of alkylaromatics, and catalytic dehydrogenation of paraffins. Of the above-mentioned net hydrogen-producing reaction systems, catalytic reforming ranks as one of the most widely employed. By virtue of its wide application and its utilization as a primary source of hydrogen for the petroleum refinery or petrochemical complex, catalytic reforming has become well known in the art of hydrocarbon conversion reaction systems.
It is well known that high quality petroleum products in the gasoline boiling range including, for example, aromatic hydrocarbons such as benzene, toluene, and the xylenes are produced by the catalytic reforming process. Typically, in such a process, a naphtha boiling range hydrocarbon fraction is passed to a reaction zone wherein it is contacted with a platinum-containing catalyst in the presence of hydrogen. Generally, the catalytic reforming reaction zone effluent, comprising gasoline boiling range hydrocarbons and hydrogen, is passed to a vapor-liquid equilibrium separation zone and is therein separated into a hydrogen-containing vapor phase and an unstabilized hydrocarbon liquid phase. A portion of the hydrogen-containing vapor phase is recycled to the reaction zone. The remaining hydrogen-containing vapor phase is available for use elsewhere in the petroleum refinery or petrochemical complex. Because the dehydrogenation of naphthenic hydrocarbons is one of the predominant reactions of the reforming process, substantial amounts of hydrogen are generated within the catalytic reforming reaction zone. Such dehydrogenation is extremely advantageous in that it provides hydrogen for use elsewhere in the refinery or petrochemical complex and results in the synthesis of valuable products, namely, aromatics. The aromatics generated within the catalytic reforming reaction zone, typically include benzene, toluene and the xylenes.
The aromatics formed within the catalytic reforming zone are the products of various concomitant reactions. These reactions include the dehydrogenation of naphthenes, the cyclization and dehydrogenation of straight chain paraffinic hydrocarbons, isomerization, and hydrogen transfer. In addition to these desirable reactions, catalytic reforming also involves hydrocracking among the products of which are relatively low molecular weight hydrocarbons including the normally gaseous hydrocarbons such as methane, ethane, propane and the butanes, substantial amounts of which are recovered in the hydrogen-containing vapor phase separated from the reforming reaction zone effluent.
As is well known in the art, the presence of hydrogen in the catalytic reforming reaction zone is beneficial in that it aids in suppressing the formation of carbonaceous compounds known as coke on the reforming catalyst. Accordingly, deposition of coke on the reforming catalyst is suppressed by high hydrogen partial pressures within the catalytic reforming reaction zone. However, the art has recognized that there are substantial aromatic yield advantages to be achieved by decreasing the hydrogen partial pressure within the catalytic reaction zone. As a result, the current trend in catalytic reforming is to increase the reaction severity within the catalytic reaction zone by operating at lower hydrogen partial pressures in order to promote the improved yields of desirable products.
The modern trend toward low hydrogen partial pressure operation of the catalytic reaction zone has however several operating disadvantages. As already mentioned, low hydrogen partial pressures tend to result in the increased deposition of coke on the reforming catalysts which results in a loss of activity. The art has addressed this problem by developing the so-called continuous catalytic reforming process wherein the reforming catalyst is continuously regenerated to maintain an acceptable level of activity. A second disadvantage of the high severity-low hydrogen partial pressure operation is the decrease in hydrogen purity of the hydrogen-containing vapor phase separated from the reaction zone effluent, typically by means of a vapor-liquid equilibrium separation zone. The reduction in hydrogen purity of the hydrogen-containing vapor phase is a result of the presence of the products formed by the hydrocracking function of the reforming catalyst emplaced within the catalytic reforming zone combined with the vapor-liquid equilibrium conditions within the vapor-liquid equilibrium separation zone. As noted above, these products typically comprise methane, ethane, propane and the butanes. The result of the presence of such low molecular weight hydrocarbons is an overall increase in the molecular weight of the hydrogen-containing vapor phase. Because a portion of the hydrogen-containing vapor phase is recycled back to the catalytic reforming reaction zone to provide at least a portion of the hydrogen necessary therefor, an increase in the molecular weight of the hydrogen-containing vapor phase necessarily results in increased utilities costs due to the increased work of compression for recycling of and increased heating and cooling of the hydrogen-containing vapor phase.
Aside from utility consumption considerations, the decrease in hydrogen purity may be disadvantageous by rendering that portion of the hydrogen-containing vapor phase not recycled to the reaction zone unusable without further processing. Accordingly, the only feasible use of the net hydrogen-containing vapor phase having low hydrogen purity may be as a fuel for the refinery or petrochemical complex fuel system. Although it admittedly has value as fuel, the hydrogen in a more pure form may be much more valuable if used in other petroleum refining or petrochemical processes. For example, it is expected that the increased demand for the distillate boiling range fuels will result in the need for increased hydrocracking capacity in many petroleum refineries. As is well known, the hydrocracking of heavy petroleum fractions consumes significant amounts of hydrogen. It would therefore be advantageous to obtain this hydrogen from the catalytic reforming process unit. Moreover, notwithstanding the general fluctuation in the deaand for petroleum crude oil, it is expected that more refractory crude oils will have to be relied on as refinery and petrochemical feedstocks. Such refractory crude oils require more hydroprocessing in converting them to usable products. In addition, it has become necessary to process the more refractory portions of crude oils, the so-called bottom of the barrel crude oil cuts. As with highly refractory crude oils, such fractions require significant hydroprocessing. Accordingly, it can be seen that the future demand for hydrogen will make its continued use as fuel for the refinery or petrochemical complex economically intolerable. It is therefore advantageous to devise a method of purifying the net hydrogen-containing vapor phase from the catalytic reformer to produce a hydrogen-rich gas stream which may be further utilized in the general upgrading of crude oils to more salable products.